Your prof may have taken issue with the fact that you took and active sentence and made is passive. Now, I don't know if that was an option to you or not, but even as a passive statement it's a little off, especially the verb:

a tyranno - no need for iste, which would suggest that very tyrannt, or that tyrannt which is a real pain in all our tails.

delere iussae sunt - I would use the perfect tense (perfect, passive, 3rd. pers. plural) of iubeo here and just the plain ol' infinitive of deleo.

Grand total: Magna cum spe illae naves a tyranno delere iussae sunt. Beware, however, that the sentence in English is an active one, so unless your prof told you to change it into the passive it is fubared upwards and sideways. So, for active: Magna cum spe tyrannus illas naves delere iussit. Nice and neat.

Actually, with commands (and ordering is a command), you would normally use ut + subjunctive.

Turendil - you used "delendam" for "to be destroyed". While in English it may be hard to distinguish the two, delendam (which actually should be delendae, since it, as an adjective, ought to have agreed with illae naves), denotes an action to be undertaken in the future. You missed out on the sequence of tenses here, since a future can't be a secondary tense when the main verb is perfect. Instead, you use the imperfect.

Chris Weimer wrote:Actually, with commands (and ordering is a command), you would normally use ut + subjunctive.

The above--less common though nevertheless acceptable--construction (iubeo + indic.) is what Wheelock's expects at the moment. The subjunctive mood and jussive clauses aren't introduced to the student until chapter 28.

I'm not sure why you think rex is a better translation for "tyrant" than tyrannus. To me, at least, there is a clear difference between a king and a tyrant, a distinction which would not have been lost on the Classical world. I don't recall (and I could be wrong) Numa Pompilius ever regarded by Classical authors as a tyrannus, yet he was well understood to be a rex.